BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to an optical branching unit installed on an optical
transmission line and to an optical transmission line.
2. Description of the Related Prior Art
[0002] Optical wavelength division multiplexing transmission (WDM transmission) is in practical
use today as a large-capacity communication technology. In WDM transmission mode,
a plurality of wavelength-multiplexed signal lights, each with its own wavelength
different from others, propagate through an optical transmission line composed of
optical fibers. For a long-distance optical transmission, optical repeaters, each
with an optical fiber amplifier, are installed at regular intervals on the optical
transmission line. In general, the gain of an optical fiber amplifier has a wavelength
dependency. As wavelength-multiplexed signal lights, which propagate a long distance,
are amplified by many optical fiber amplifiers, the intensities of signal lights differ
greatly, degrading the reception S/N ratio. To solve this problem, each signal light
may be assigned a unique intensity when transmitted from the transmitter. However,
when the intensity of a particular signal light is too high, a nonlinear degradation
results, decreasing the transmission characteristics. Thus, an optical repeater on
an optical transmission line usually has an optical gain equalizer which compensates
for the wavelength dependency of the gain of an optical fiber amplifier. Some of several
optical repeaters has an optical gain equalizer in other systems. However, the problem
is that, in some cases, an optical transmission line has many optical branch lines
each with its own requirements. For example, in an optical transmission system having
optical branch lines, optical repeaters, each with an optical fiber amplifier, are
not always installed at regular intervals. In this case, if optical repeaters have
an optical fiber amplifier and an optical gain equalizer with some specific characteristics,
it is difficult to optimally design an optical transmission system with many optical
branch lines. On the other hand, changing the gains of the optical fiber amplifiers
and adjusting the characteristics of optical gain equalizers according to the conditions
of each optical branch line in the whole optical transmission system are very costly.
SUMMARY OF THE INVENTION
[0003] In view of the foregoing, it is an object of the preferred embodiments of the present
invention to provide, for use on an optical transmission line, with branch lines,
an optical branching unit with a configuration that allows the signal light gains
to be equalized and an optimal transmission system to be easily designed.
[0004] The optical branching unit according to the present invention comprises a gain equalizer
adjusting levels of a plurality of signal lights. The gain equalizer equalizes the
levels of the plurality of signal lights. This gain equalizer compensates for the
wavelength dependency of the gains of one or more optical fiber amplifiers installed
on an optical transmission line. One or more gain equalizers maybe installed before,
after, or before and after, the branch point of the optical branching unit. A plurality
of gain equalizers, if used, may adjust the levels of signal lights each in its own
wavelength band. On the other hand, the gain equalizers may adjust the levels of signal
lights whose wavelength bands overlap partially or completely. The optical transmission
line according to the present invention comprises one or more optical branching units
and at least one of the optical branching units comprises a gain equalizer adjusting
the signal light levels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Preferred features of the present invention will now be described, by way of example
only, with reference to the accompanying drawings, in which:-
FIG. 1 is a block diagram showing an example of the configuration of an optical branching
unit according to the present invention and the characteristics.
FIG. 2 is a block diagram showing another example of the configuration of the optical
branching unit according to the present invention.
FIG. 3 is a block diagram showing still another example of the configuration of the
optical branching unit according to the present invention.
FIG. 4 is a block diagram showing still another example of the configuration of the
optical branching unit according to the present invention.
FIG. 5 is a block diagram showing still another example of the configuration of the
optical branching unit according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0006] Referring to FIG. 1, an optical branching unit 1 has an optical gain equalizer 2
on each of the branch lines. The wavelength multiplexed optical signal lights sent
from station A have approximately the same level before entering an optical fiber
amplifier 3. The signal lights propagate through the optical transmission line and
pass through one or more optical fiber amplifiers 3. Because the amplification factor
of the optical fiber amplifier 3 has a wavelength dependency, the level difference
rises and grows each time the signal lights pass through the optical fiber amplifiers
3. For a signal light with a wavelength whose amplification factor is low, the level
ratio between the spontaneous emission light generated from the optical fiber amplifier
and the signal light is small, meaning that reception errors will likely occur. The
signal lights are branched into two at the optical branching unit 1. The branched
lights pass through the optical gain equalizers 2 and the optical fiber amplifiers
3. One of them is sent to station B, while the other is sent to station C. At this
time, the optical gain equalizers 2 installed in the optical branching unit equalize
the gains of the signal lights to make the signal light levels equal at the start
of the optical branch lines. Therefore, even if the requirements of the optical branching
lines differ, an optical transmission system may be designed more easily. Each of
the two optical gain equalizers 2 may have its own gain equalization characteristics
according to the requirements of the branch line. In addition, the optical gain equalizer
2 may be installed in one of the branch lines only.
[0007] Referring to FIG. 2, one optical gain equalizer 2 is installed before the branch
point of the optical branching unit 1. This optical branching unit 1, with only one
optical gain equalizer 2, is less costly but lacks design flexibility when designing
an optical transmission system.
[0008] Referring to FIGS. 3 and 4, the optical branching unit 1 has a plurality of optical
gain equalizers 2. When more signal lights are wavelength-multiplexed and the signal
light wavelength band becomes wide, all signal lights cannot be processed by one optical
gain equalizer. In such a case, half of the signal lights are processed, for example,
by one optical gain equalizer 2 with the rest processed by the other optical gain
equalizer 2. Some wavelength bands processed by these optical gain equalizers do not
overlap, while other wavelength bands partially overlap. Or, though the wavelength
areas processed by both optical gain equalizers are the same, each may have its own
loss characteristics (gain equalization characteristics). In this case, the characteristics
composed by those characteristics compensate for the gain difference of the signal
lights.
[0009] Referring to FIG. 5, two optical gain equalizers 2 are provided after, and one optical
gain equalizer 2 is provided before, the branch point of the optical branching unit
1.
[0010] When an optical transmission line has a plurality of optical branching units, all
optical branching units, or only some of them, may have optical gain equalizers.
[0011] The optical gain equalizer 2 installed in the optical branching unit 1 equalizes
the signal light levels completely or makes a signal level difference among the signal
lights according to the status of the optical branch lines. In some cases, optical
gain equalizers are installed in one optical branch line only. The wavelength-multiplexed
signal lights may be, for example, in the 1.55-micron wavelength band with the signal
light wavelength interval of 0.8 nm to 1.0 nm. The optical branching unit may be a
known device such as a normal optical fiber coupler. The optical gain equalizer is
known. For this device, a fiber grating, composed of an optical fiber with a periodically-changing
refractive index, is used. Adjusting the grating interval, grating length, and soon
allows attenuation characteristics to be composed that compensate for the wavelength
dependency of an optical amplifier. In the above figures, the signal lights (T1∼8)
entering the optical branching unit are represented by eight lines, and the branched
signal lights (B1∼8) by four lines. In practice, eight signal lights (T1∼8) enter
the optical branching unit and each branched light (B1∼8) is composed of eight signal
lights. As described above, the optical branching unit according to the present invention
comprises one or more optical gain equalizers, thus facilitating the gain equalization
of the whole optical transmission line system having optical branch lines. Figs. 1
to 5 are very simplified figures. Transmission lines including branch lines have their
own repeaters and equalizers in order to relay and equalize signal lights.
[0012] While the present invention has been described in connection with certain preferred
embodiments, it is to be understood that the subject-matter encompassed by the present
invention is not limited to those specific embodiments. On the contrary, it is intended
to include all alternatives, modifications, and equivalents as can be included within
the scope of the following claims.
[0013] Each feature disclosed in this specification (which term includes the claims) and/or
shown in the drawings may be incorporated in the invention independently of other
disclosed and/or illustrated features.
[0014] The text of the abstract filed herewith is repeated here as part of the specification.
[0015] An optical branching unit for installation on an optical transmission line comprises
an optical gain equalizer. The optical gain equalizer equalizes the levels of wavelength-multiplexed
signal lights amplified by an optical amplifier. Because the levels of the signal
lights propagated through optical branch lines become equalized at a branching point,
the design of the optical transmission system with the optical branch lines is simplified.
The optical branching unit may comprise a plurality of optical gain equalizers.
1. An optical branching unit for installation on an optical transmission line, said optical
branching unit comprising a gain equalizer adjusting levels of a plurality of signal
lights.
2. The optical branching unit according to claim 1, wherein said gain equalizer equalizes
levels of the plurality of signal lights.
3. The optical branching unit according to claim 1, wherein said gain equalizer compensates
for a wavelength dependency of a gain of at least one optical fiber amplifier installed
on the optical transmission line.
4. The optical branching unit according to claim 1, wherein at least one gain equalizer
is installed before or after or both before and after a branch point of said optical
branching unit.
5. The optical branching unit according to claim 1, wherein said gain equalizer is installed
only on one of optical branch lines after a branch point of said optical branching
unit.
6. The optical branching unit according to claim 1, wherein a plurality of gain equalizers
are installed, said plurality of gain equalizers adjusting the levels of the signal
lights in different wavelength bands or in partially- or completely-overlapped wavelength
bands.
7. The optical branching unit according to any one of the preceding claims, in combination
with the optical transmission line.